The mammalian pancreas develops from the embryonic foregut bud. As the embryonic buds grow, a ductal system develops by branching morphogenesis. After the ventral and dorsal anlage fuse, the new organ grows and matures into two interlocked structures, the exocrine system and the endocrine system. The majority of the pancreas is composed of acinar cells that produce digestive enzymes. The endocrine system includes β-cells, which produce insulin, α-cells, which produce glucagon, and δ-cells, which produce somatostatin. The endocrine cells are organized into clusters called islets.
Animal research has shown at least two mechanisms of β-cell formation: neogenesis from ductal precursor cells and replication of mature β-cells. Replication of differentiated β-cells is maintained postnatally into adulthood. Replication of β-cells is accelerated by an increased demand for insulin, for example, as a result of high glucose infusion, partial pancreatectomy, and during gestation. Under these conditions, β-cells mass quickly increases through both cell hypertrophy (enlargement of volume of individual cells) and hyperplasia (increase in the number of β-cells).
In Type I or insulin dependent diabetes mellitus (IDDM) there is a clear reduction in the number of β-cells due to an autoimmune attack against the β-cells. Eisenbarth, N. Eng. J. Med. 314:1360–1368 (1986). A treatment for Type I diabetes could include increasing in the number of β-cells in a subject suffering from Type I diabetes. Bonner-Weir, Endocrin. 141:1926–1929 (2000).
Another treatment for diabetes using islet cells involves grafting pancreatic tissue from immune matched donors into transplant recipients. Typically, transplant recipients are required to receive immunosuppressant therapy to prevent rejection of the transplanted organ. Recently developed immunosuppressant regimens have improved the results of clinical islet transplantation in humans. While the technique remains experimental, if islet cell transplants can perform the same function as whole organ pancreas grafts, this much simpler surgical procedure would play an important role in the treatment of diabetes.
Although the transplantation of human islets shows promise as a powerful treatment for diabetes, a number of impediments exist that presently limit the utility of this procedure. One significant impediment is the inability to produce sufficient numbers of islet cells for use in the procedure. Presently, the process used to obtain islets for transplantation typically involves isolation of pancreatic tissue, enzymatic digestion of the pancreatic tissue to liberate the individual cells from the surrounding tissue, and the use of a gradient centrifugation purification technique. The gradient centrifugation purification technique is well known in the art and is performed by many islet transplant centers. Unfortunately, the yield of islets from a single pancreas treated with the standard procedure is usually insufficient for transplantation. Accordingly, alternatives to this procedure have been sought and developed.
A method to selectively propagate intermediate stage pancreatic stem cells has been developed. Briefly, after isolation from donor pancreas, cells are allowed to recover in high serum media and are then switched to low serum media to promote the growth of intermediate stage pancreatic stem cells. These cells can be matured into insulin secreting cell aggregates by growth to confluence on conditioned culture dishes. However, these aggregated cells are of limited clinical use, because if transplanted directly, the cells may raise a detrimental immune response in the host.